(a) Field of the Invention
The present invention relates to a vari-focal lens system usable in multiple focal length conditions and to be used in photographic lens systems, video camera lens systems, electronic still camera lens systems, etc.
(b) Description of the Prior Art
As the conventional examples of lens systems easily usable in multiple focal length conditions in photographic lens systems, etc., there are already known the lens systems disclosed by Japanese Unexamined Published Patent Application No. 206514/62 and Japanese Unexamined Published Patent Application No. 177513/62.
Each of these lens systems is so adapted as to change focal length thereof by inserting one of two auxiliary lens units different from each other after a master lens system. Especially, the lens system disclosed by the former patent Application is so designed as to permit using only the master lens system in a focal length condition and is usable in three focal length conditions. Since these conventional lens systems are so designed as to require removing out of the optical axis the auxiliary lens unit already attached to the master lens system before inserting the other auxiliary lens unit, these lens systems require rather long time for switching the auxiliary lens units and have a fear to miss good shutter operating timing. If an attempt is made to overlap the removing time with the inserting time of the auxiliary lens units for correcting the defect described above, it is necessary to use a means for preventing both the lens units from hindering movements by each other, thereby posing restrictions on spatial arrangement and movement courses of the auxiliary lens units as well as the mechanism for driving the auxiliary lens units. As a result, other defects will be produced to lose compactness, increase weight and enhance manufacturing costs of the vari-focal lens systems.
Further, the conventional vari-focal lens systems require an auxiliary lens unit for each focal length condition. When the number of the focal length conditions is increased, the refuge spaces and switching mechanisms for the auxiliary lens units are increased, thereby enlarging the lens systems, increasing weights, enhancing manufacturing costs thereof, and finally making it substantially difficult to obtain four or more focal length conditions.
Furthermore, in a case where an attempt is made to obtain a shorter focal length condition by attaching an auxiliary lens unit to a master lens system, a total length of a vari-focal lens system will be prolonged as compared with a case where the focal length condition is obtained with an independent lens system. Moreover, when a focal length which is the same as the above-mentioned focal length condition is obtained by attaching, for prolonging focal length, an auxiliary lens unit to a master lens system having a focal length shorter than that of the above-mentioned master lens system, the former master lens system proper has a longer length as measured from the foremost surface of the lens system to the film surface and, therefore, it is difficult to shorten the total length of the vari-focal lens system.